The present invention pertains to the electric power art and, more particularly, to an improved overvoltage protector.
Overvoltage protectors are well known in the prior art. FIG. 1 is a schematic diagram of an overvoltage protector for use in a commercial airplane. Here, the airplane's generator 10 produces at its output terminals 12, 14 an AC voltage having a nominal peak value of 162 volts at a frequency of 400 hertz. The output from the generator 10 is passed through a feeder inductance 16, here represented as a lumped inductance, to the feeder line 20. Feeder line 20, as well as generator 10, are subject to induced high voltage transients, such as may occur due to a lightning strike on the aircraft. The high induced voltages may result in damage to the aircraft's electrical equipment (not shown). Thus, a lightning protector, indicated generally at 24, is provided between the generator's terminals 12, 14.
The lightning protector 24 includes a voltage sensing device 30 which is wired directly across the generator's terminals 12, 14 and includes internal circuitry (not shown) which senses the voltage on the line. If the peak voltage exceeds 250 volts, the voltage sensing device 30 produces a trigger signal at its outputs 32, 34. While a detailed schematic diagram of the voltage sensing device 30 is not shown herein, such circuits are well known to the prior art.
The trigger signals produced at the outputs 32, 34 of the voltage sensing device 30 are coupled via transformers 40, 42, respectively, to the gate-cathode connections of a pair of silicon controlled rectifiers 50, 52. The silicon controlled rectifiers 50, 52 are connected in parallel and in reverse polarity such that the anode of one rectifier connects to the cathode of the other. As shown, the common connection of the anode of the first silicon controlled rectifier 50 with the cathode of the second 52 connects to the second generator terminal 14.
The common connection formed by the cathode of the first SCR 50 and the anode of the second SCR 52 connects through a network, indicated generally at 60 formed by the parallel connection of a capacitor 62 and a resistor 64, to the feeder line 20. Also shown in FIG. 1 is a lumped capacitance 70 connected across the generator terminals 12, 14 which represents the total shunt capacitance due to equipment tied to the line.
Operation of the prior art voltage protector 24 shown in FIG. 1 may be understood as follows. During normal operation of the system, the voltage produced by the generator 10 at its output terminals 12, 14 does not rise to the threshold of the voltage sensing circuit 30 and thus the SCR's 50, 52 are biased off. Capacitor 62, thus, is normally discharged and, in effect, is connected only to the generator feeder line 20.
If a transient voltage, such as may be caused by lightning, is induced into the generator feeder 20, it is detected by the voltage sensing device 30 when it has risen to 250 volts. A trigger voltage is then applied to the gates of both SCR's 50 and 52. One of these will be switched on depending on whether the feeder voltage is positive or negative. That is, SCR 50 is switched on for negative feeder voltages whereas SCR 52 will switch on for positive feeder voltages. The switching on of one of the devices 50, 52 causes one end of capacitor 62 to be clamped near the potential at generator line 14, which is commonly airplane ground.
At the instant when the overvoltage protector 24 fires, capacitor 70 will be charged to 250 volts. A resultant large, very fast rising current spike flows from capacitor 70 to capacitor 62 tending to equalize their respective voltages. Also, current from the lightning protector 24 must briefly replace a large part of the load current which had previously been flowing in the feeder inductance 16. Thus, very high currents exist at the time of protector firing.
The capacitance of capacitor 62 must be large enough to absorb the current that flows to the protected point on the generator feeder due to the lightning induced transient without allowing that protected point to rise to a voltage which might damage load equipment.
The subject matter of the instant invention includes the recognition of a problem with the circuit of FIG. 1 which can result in damage to one or both of the SCR's 50, 52. It has been discovered that damage to the SCR's can be caused if the lightning protector 24 is triggered at a time when capacitor 62 retains a residual charge from a previous cycle of operation. As an example, assume that a positive transient causes SCR 52 to be triggered shortly after zero crossing of a positive half cycle of the 400 Hertz power frequency. Capacitor 62 will absorb the transient and, if not charged to a higher voltage by the transient, will charge to the 162 volt peak of the 400 Hertz AC voltage. SCR 52 turns off when the forward flow of current ceases. The time constant of capacitor 62 and resistor 64 is long, so that if a second transient occurs within the time of a few cycles of the 400 Hertz, capacitor 62 will still retain most of the charge from the first transient suppression. If the second transient is negative, the instantaneous voltage at the cathode of SCR 50 relative to ground terminal 14 when it is triggered will be as great as -410 volts, i.e., -160 volts due to the residual charge on capacitor 62 plus -250 volts on the feeder. This high voltage may result in a destructive current surge through SCR 50. If the negative transient also occurs during a negative half cycle of the 400 Hertz power waveform, the probability and severity of damage may be increased due to load current displacement and a high final reverse charge on capacitor 62.
Further, lightning does not always occur in single strokes. Should a re-strike occur, the capability of capacitor 62 to absorb the feeder induced transient is reduced if the second transient is of the same polarity as the residual charge on capacitor 62.
In alternate prior art transient suppression circuits, a voltage responsive resistance is placed directly across the line. A voltage responsive resistance has a non-linear voltage response, exhibiting a relatively high impedance at all voltages below a certain threshold and a relatively low impedance for any applied voltage above the threshold. Thus, the threshold of the device is selected to be above the peak of the nominal generator voltage on the line, thereby maintaining the device in a high impedance state. Upon the occurrence of a high voltage transient, the device switches to its low impedance state, suppressing the transient to ground.
A fundamental problem with such prior art constructions is that any voltage responsive resistance device has a small constant resistance in series with it. For high peak transient currents, such as produced by a lightning strike, the voltage drop across the constant resistance is sufficiently high such that when it is added to the threshold voltage of the device the resulting voltage across the line is excessive.
Thus, there is a need in the prior art for an overvoltage protector which does not suffer the above-described deficiencies.